99 



have here recorded, we see, t hat within the tropics, 

 the moschettoes and zancudoes do not rise on the 

 slope of the Cordilleras* toward the temperate 

 region, where the mean heat is below 19° or 20° 

 centigrade and that, with few exceptions, they 

 shun the black waters, and dry and unwooded 

 spots J. The atmosphere swarms with them much 

 more in the Upper, than in the Lower Oroonoko, 

 because in the former the river is surrounded 

 with thick forests on it's banks, and the skirts 

 of the forests are not separated from the river by 

 a barren and extensive beach. The moschet- 

 toes diminish on the New Continent with the 



* The culex pipiens of Europe does not, like the culex of 

 the torrid zone, shun mountainous places. Mr. Giesecke 

 suffered from these insects in Greenland, at Disco, in latitude 

 70*. They are found in Lapland in summer, at three or four 

 hundred toises high, and at a temperature of 11° or 12° cent. 

 They give to the alpine region a character of movement and 

 life, which Mr. Wahlenbcrg seems to regret that he did not 

 find in the Alps of Switzerland, " ubi culices apesque nullas cho- 

 reas agunt." See the work of this traveller, de Vegetutione et 

 Clim. Helvet, septentr. p. xxxv. 



+ Below 15'2° or 16° Reaumur. (This is the mean tem- 

 perature of Montpellier and Rome.) 



% Trifling modifications in the waters, or In the air, often 

 appear to prevent the development of the moschettoes. Mr. 

 Bowdich remarks, that there are none at Coomassie, in the 

 kingdom of the Ashantees, though the town is surrounded by 

 marshes, (Mission to Ashantee, 1819, p. 321,) and though the 

 thermometer keeps up between seventeen and twenty-eight 

 centesimal degrees,day and night (13'6° and 22*4 6 of Reaumur.) 



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